Integrated-Optics-Based Stress-Optic Phase Modulator and Method for Forming

ABSTRACT

A phase controller for controlling the phase of a light signal in a surface waveguide and a method for its fabrication are disclosed. The phase controller controls the phase of the light signal by inducing stress in the waveguide structure, thereby controlling the refractive indices of at least some of its constituent layers. The phase controller includes a phase-control element formed on topographic features of the top cladding of the waveguide, where these features (1) provide a shape to the phase-control element that matches the shape of the mode field of the light signal and (2) give rise to stress-concentration points that focus and direct induced stress into specific regions of the waveguide structure, thereby providing highly efficient phase control. As a result, the phase controller can operate at a lower voltage, lower power, and/or over a shorter interaction length than integrated-optic phase controllers of the prior art.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 15/875,340 filed Jan. 19, 2018 (Attorney Docket:142-029US1), which claims the benefit of U.S. Provisional ApplicationSer. No. 62/448,112, filed Jan. 19, 2017, entitled “Phase ControllerComprising a Stress-Inducing Phase Modulator” (Attorney Docket:142-029PR1), each of which is incorporated by reference, in itsentirety, as if set forth at length herein. If there are anycontradictions or inconsistencies in language between this applicationand one or more of the cases that have been incorporated by referencethat might affect the interpretation of the claims in this case, theclaims in this case should be interpreted to be consistent with thelanguage in this case.

BACKGROUND OF THE INVENTION

A Planar Lightwave Circuit (PLC) is an optical system comprising one ormore integrated-optics waveguides that are integrated on the surface ofa substrate, where the waveguides are typically combined to providecomplex optical functionality. These “surface waveguides” typicallyinclude a core of a first material that is surrounded by a secondmaterial having a refractive index that is lower than that of the firstmaterial. The change in refractive index at the interface between thematerials enables reflection of light propagating through the core,thereby guiding the light along the length of the surface waveguide.

PLC-based devices and systems have made significant impact in manyapplications, such as optical communications systems, sensor platforms,solid-state projection systems, and the like. Surface-waveguidetechnology satisfies a need in these systems for small-sized, reliableoptical circuit components that can provide functional control over aplurality of optical signals propagating through a system. Examplesinclude simple devices (e.g., 1×2 and 2×2 optical switches, Mach-Zehnderinterferometer-based sensors, etc.), as well as more complex,matrix-based systems having multiple surface waveguide elements and manyinput and output ports (e.g., wavelength add-drop multiplexers,cross-connects, wavelength combiners, etc.).

Common to most of these systems is a need for a switching element.Historically, the most common switching elements suitable for use in aPLC are based on a device known as a thermo-optic (TO) phase controller.A TO phase controller takes advantage of the fact that the refractiveindex (i.e., the speed of light in a material) of glass istemperature-dependent (referred to as the thermo-optic effect) byincluding a thin-film heater that is disposed on the top of the uppercladding of a surface waveguide. Electric current passed through theheater generates heat that propagates into the cladding and corematerials, changing their temperature and, thus, their refractiveindices. TO phase controllers have demonstrated induced phase changes aslarge as 2 a.

To form an optical switching element, a TO phase controller is typicallyincluded in a surface waveguide element, such as a Mach-Zehnderinterferometer (MZI). In an MZI switch arrangement, an input opticalsignal is split into two equal parts that propagate down a pair ofsubstantially identical paths (i.e., arms) to a junction where they arethen recombined into an output signal. One of the arms incorporates a TOphase controller that controls the phase of the light in that arm. Byimparting a phase difference of π between the light-signal parts in thearms, the two signals destructively interfere when recombined, therebycanceling each other out to result in a zero-power output signal. Whenthe phase difference between the light-signal parts is 0 (or n*2π, wheren is an integer), the two signals recombine constructively resulting ina full-power output signal.

Unfortunately, prior-art PLC-based switching elements have disadvantagesthat have, thus far, limited their adoption in many applications. First,TO phase controllers consume a great deal of power. Further, in additionto heating the core and cladding materials directly below the heaterelement, heat from the thin-film heater also diffuses laterally in theglass, which can lead to thermal crosstalk between adjacent surfacewaveguides. Still further, glass has a low thermal conductivitycoefficient, which results in heating and cooling times that are long(typically, on the order of milliseconds). Thermal crosstalk also limitsthe density with which heating elements can be formed, limiting thenumber of TO phase controllers that can be included on a single chip. Asa result, TO phase controllers are poorly suited for many applications.

More recently, the photo-elastic effect has been exploited as analternative to thermo-optic tuning of the refractive index of thematerials of a surface waveguide. Phase shifting of a light signal insurface waveguides based on the photo-elastic effect was disclosed, forexample, by S. Donati, et al., in “Piezoelectric Actuation ofSilica-on-Silicon Waveguide Devices,” published in IEEE PhotonicsTechnologies Letters, Vol. 10, pp. 1428-1430 (1998), and by Tsia, etal., in “Electrical Tuning of Birefringence in Silicon Waveguides,” App.Phys. Lett., Vol. 92, 061109 (2008), and in U.S. Pat. Nos. 9,221,074 and9,764,352, each of which is incorporated herein by reference. Whilephase shifting on the order of a microsecond with low power dissipationwas demonstrated, the efficiency with which a phase change could beinduced in the constituent layers (particularly the core layer) of thesurface waveguides was poor. As a result, very high voltages and largeinteraction lengths were required, which limits the utility of prior-artphoto-elastic-based phase tuning in practical PLC systems.

The need for an efficient integrated-optics phase tuning technology thatenables fast, low-power-consumption operation remains, as yet, unmet inthe prior art.

SUMMARY OF THE INVENTION

The present invention enables photo-elastic-based phase control of alight signal propagating in a surface waveguide with higher efficiencythan the prior art. Embodiments of the present invention areparticularly well suited for use in applications such astelecommunications, data communications, projection systems, andsensors.

Like prior-art stress-optic phase controllers, embodiments of thepresent invention employ a phase-control element disposed on atopological feature of a surface waveguide to induce laterally andvertically directed stress into its constituent layers. In theprior-art, these topological features are either formed as part of theupper cladding of the waveguide or are due to deposition of the uppercladding layer over the structure of the core itself.

In contrast to the prior art, in embodiments of the present invention,the topological feature is a projection of upper cladding material thatis formed by partially etching the lower cladding to create a ridge-likeprojection and overcoating this projection via conformal deposition ofthe upper cladding material. As a result, the present invention providesindependent control over the geometry of the waveguide core, thethicknesses of the upper and lower cladding layers, and the relativepositions of the waveguide core and the phase-control element, whichenables application of the present invention to virtually any surfacewaveguide structure and technology. Embodiments in accordance with thepresent invention, therefore, are suitable for use with virtually anysurface waveguide technology, including ridge waveguides, channelwaveguides, low-index-contrast waveguides, high-index-contrastwaveguides, surface waveguides having homogeneous core structures,surface waveguides having cores comprising multiple layers of differentmaterials, and surface waveguides formed in a wide range of materialsystems.

Furthermore, the independence of the waveguide core and the claddinglayer structure enables the position of the phase-control element,relative to the waveguide core, to be selected to facilitate theefficiency with which it induces stress into the core. For example,stress-concentration points in the phase-control element can reside atthe same height or below the top surface of the waveguide core, therebyenabling refractive-index changes in the light-guiding materials to bemore efficiently induced.

An illustrative embodiment of the present invention is a phasecontroller comprising a stress-optic phase-control element disposed on aprojection formed by partially etching the lower core of a waveguide todefine a ridge and conformally depositing the upper core layer over theridge. The phase-control element is formed on the projection such thatits shape substantially matches the shape of the optical mode supportedby the waveguide core. In addition, the phase-control element extendsdown the sides of the projection to realize stress-concentration pointsthat are below the top surface of the core of the waveguide and can moreeffectively impart stress into the core materials.

In addition, the projection on which the phase-control element isdisposed has a shape that substantially matches the shape of the opticalmode supported by the waveguide. As a result, the phase-control elementis in close proximity to a greater portion of the perimeter of the modefield, thereby increasing its effectiveness in inducing a desired phasedelay.

In some embodiments, the depth to which the lower cladding is etched andthe thicknesses of the top cladding layer and the phase-control elementare selected to realize a structure in which the top of thephase-control element is substantially aligned with the top of the core.

An embodiment of the present invention is a phase controller comprising:a surface waveguide (302) disposed on a substrate (116), the surfacewaveguide including a lower cladding (306), a core (308), and an uppercladding (310) comprising a projection (316) and a field region (520),wherein the surface waveguide supports a mode field (324) having a firstshape; and a phase-control (PC) element (304) disposed on at least aportion of the projection, wherein the PC element comprises a firstelectrode (312-1), a second electrode (312-2), and a piezoelectric layer(314) that is between the first and second electrodes; wherein the corehas a first surface (514) that defines a first plane (P1), and whereinthe field region has a second surface (526) that defines a second plane(P3), and further wherein the second plane is at or below the height ofthe first plane; wherein the PC element includes a firststress-concentration point (SP1A) that is at or below the height of thefirst plane; and wherein the PC element has a shape that issubstantially matched to the shape of the mode field.

Another embodiment of the present invention is a method for forming aphase controller (104), the method comprising: providing a waveguide(302) that is operative for conveying a light signal (112) having a modefield (324) having a first shape, the waveguide comprising: a firstcladding (306) having a first field region (516) and a spine (530) thatprojects from the first field region; a core (308) that is disposed onthe spine, wherein the core has a first surface (514) that defines afirst plane (P1), and wherein the core and the spine collectively definea ridge (518); and a second cladding (310) that includes a second fieldregion (520) and a projection (316) that projects from the second fieldregion, wherein the second cladding is conformally disposed on the firstfield region and the ridge such that the ridge and the second claddingcollectively define the projection, and wherein the second field regionhas a second surface (524) that defines a second plane (P2); and forminga phase-control (PC) element (304) comprising: a first electrode (312-1)that is in direct contact with the third surface; a second electrode(312-2) that is distal to the third surface; and a piezoelectric layer(314) that is between the first and second electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of a top view of a PLC-based opticalswitch comprising an integrated-optic-based stress-optic phasecontroller in accordance with an illustrative embodiment of the presentinvention.

FIG. 2 depicts a schematic drawing of a cross-sectional view of astress-optic phase controller in accordance with the prior art.

FIG. 3 depicts a schematic drawing of a cross-sectional view ofstress-optics phase controller in accordance with the illustrativeembodiment of the present invention.

FIG. 4 depicts operations of a method suitable for forming astress-optic phase controller in accordance with the illustrativeembodiment.

FIGS. 5A-D depict schematic drawings of cross-sectional views of phasecontroller 104 at different points in its fabrication.

FIG. 6 depicts a simulation of induced refractive index change versusover-etch depth for a phase controller in accordance with the presentinvention.

FIG. 7 depicts a schematic drawing of a top view of a phase controllerin accordance with a first alternative embodiment of the presentinvention.

FIG. 8 depicts a schematic drawing of a top view of a phase controllerin accordance with a second alternative embodiment of the presentinvention.

DETAILED DESCRIPTION

For the purposes of the present Specification, including the appendedclaims, the following terms are defined:

-   -   “Disposed on” and “Formed on” are defined as “exists on” an        underlying material or layer either in direct physical contact        or with one or more intervening layers. For example, if a        material is described to be “disposed (or grown) on a        substrate,” this can mean that either (1) the material is in        intimate contact with the substrate; or (2) the material is in        contact with one or more layers that already reside on the        substrate. It should be noted that a conformal layer is        considered disposed on each surface of a structure to which it        conforms;    -   “integrated-optics waveguide,” “surface waveguide,” and        “waveguide” are used interchangeably and defined to mean a        PLC-based waveguiding structure comprising a lower cladding        layer, a core, and an upper cladding layer formed on the surface        of a substrate;    -   “mode-field diameter” is defined as the distance from the center        of a guided optical mode at which the electric and magnetic        field strengths are reduced to 1/e of their maximum values        (typically located at the center of the mode).

FIG. 1 depicts a schematic drawing of a top view of a PLC-based opticalswitch comprising an integrated-optic-based stress-optic phasecontroller in accordance with an illustrative embodiment of the presentinvention. Switch 100 includes Mach-Zehnder Interferometer 102 and phasecontroller 104. Although the illustrative embodiment is a PLC comprisingan integrated-optics switch, it will be clear to one skilled in the art,after reading this Specification, how to specify, make, and usealternative embodiments of the present invention that include any PLCsystem and/or surface waveguide layout.

Mach-Zehnder Interferometer (MZI) 102 includes input waveguide 106, arms108A and 108B, and output waveguide 110, which are arranged such that,as light signal 112 propagates through input waveguide 106, its opticalenergy is split equally into light signals 112A and 112B, whichpropagate through arms 108A and 108B, respectively, to output waveguide110 where they combine to form output signal 114.

In the depicted example, the lengths of arms 108A and 108B are designedsuch that light signals 112A and 112B are in phase and constructivelycombine at output waveguide 110 when phase controller 104 is in itsquiescent state. As a result, when no control voltage is applied tophase controller 104, the intensity of output signal 114 issubstantially equal to the intensity of input signal 112 (neglectingpropagation loss in the waveguides of MZI 102).

When phase controller 104 is activated, however, it induces a stress inthe waveguide structure of arm 108B, which gives rise to a change in thespeed at which light signal 112B travels through the arm. The magnitudeof this induced stress is controlled to control the phase differencebetween light signals 112A and 112B when they recombine, therebyenabling control over the magnitude of output signal 114.

As noted above, phase control for a light signal in a surface waveguidebased on the photo-elastic effect is known in the prior art; however,known stress-optic phase controllers are inadequate to the task in manyapplications due to the fact that they do not efficiently induce stresswhere needed in the waveguide structure.

FIG. 2 depicts a schematic drawing of a cross-sectional view of astress-optic phase controller in accordance with the prior art. Phasecontroller 200 includes waveguide structure 202 and stress-opticphase-control (PC) element 204. The view shown in FIG. 2 is analogous tothe view through line a-a of FIG. 1.

Waveguide structure 202 is a high-index-contrast waveguide that includeslower cladding 208, core 210, and upper cladding 212, which are disposedon underlying substrate 206. As discussed in U.S. Pat. No. 9,764,352,the upper and lower cladding layers and the core are configured totightly confine a light signal propagating through the waveguidestructure such that its mode profile extends only slightly into thecladding layers. In the depicted example, the cladding layers are layersof silicon dioxide and the core is a multi-layer core comprising lowerand upper layers of silicon nitride and a central layer of silicondioxide. Upper cladding 212 includes projection 218, which is disposedabove core 210.

In the depicted example, waveguide structure 202 supports an opticalmode having mode field 224, which is substantially elliptical.

PC element 204 includes electrode layers 214-1 and 214-2, piezoelectriclayer 216. Piezoelectric layer 216 resides between electrode layers214-1 and 214-2 and all three layers are formed such that they aredisposed on at least the top and side surfaces of projection 218 (i.e.,top surface 220 and side surfaces 222). As a result, PC element 224includes stress concentration points 226, which are located at its sharpinterior and exterior corners.

The geometry of PC element 204 enables it to impart and control bothhorizontal and vertical stress tensors in the waveguide layers ofwaveguide structure 202.

Furthermore, PC element 204 is formed in a recessed region of uppercladding 212 such that the PC element is located near enough to core 210to enable it to induce stress in the core and upper cladding materials.The distribution of the lateral and vertical stresses induced in thewaveguide layers is controlled by controlling the ratio of the width,w1, of projection 218 to the separation distance, d1, between core 210and the bottom of the projection. In fact, it was found that by keepingthis ratio within the range of approximately 2.5:1 to approximately 6:1,a measure of control over the modal birefringence in a surface waveguidecould be achieved.

It should be noted that in phase controller 200, the entirety ofphase-control element 204, including its stress concentration points226, is located above core 210. In addition, the shape of phase-controlelement is that of a partial rectangle. As a result, the shape of thephase-control element is not matched to the optical mode that it isdesigned to affect. For the purposes of this Specification, includingthe appended claims, a shape is defined as “matched” to a mode fieldover a given distance if the separation between them varies by less than20% of the mode-field diameter over that distance. Specifically, theseparation between phase-controller 204 and mode field 224 increasesdramatically near the interior corners of lower electrode 214-1, ascompared to the small separation between the top of mode field 224 andPC element 204 at the center of the optical mode.

It is an aspect of the present invention, however, that the efficiencyof a stress-optic phase controller is improved by forming itsphase-control element on a projection above the core of a waveguide suchthat:

-   -   i. the shape of the phase-control element matches the shape of        the mode field of light signal being controlled; and    -   ii. at least some of the stress concentration points of the        phase controller reside at or below at least a portion of the        core of the waveguide with which it is operatively coupled.

As a result, phase controllers of the present invention more effectivelyinduce refractive-index changes in the light-guiding materials, whichenable it to induce a 2π phase change in a light signal at a lower drivevoltage and/or over a shorter interaction length.

It is another aspect of the present invention that these desiredfeatures of a phase controller can be readily achieved via a processthat forms the projection by partially etching the lower cladding tocreate a ridge-like projection and overcoating this projection viaconformal deposition of the upper cladding material. Furthermore,fabrication processes in accordance with the present invention provideindependent control over the geometry of the waveguide core, thethicknesses of the upper and lower cladding layers, and the relativepositions of the waveguide core and the phase-control element. As aresult, embodiments of the present invention are applicable to virtuallyany surface waveguide structure and technology.

Still further, the independence of the geometry of the waveguidestructure and phase-control element enables each to be fabricated toattain high functionality without degrading the functionality of theother. For example, the phase controller can be located at any desiredheight that enables it to efficiently induce stress in the core. In someprior-art phase controllers, such as those disclosed by Tsia, et al.,the position and shape of the phase-control element is inextricablylinked to the shape of the waveguide core, since it is the shape of thecore that determines the topology on which the phase-control element isformed.

It should be noted that relative terms, such as “above” and “below” areused herein to describe physical relationships of features disposed on asubstrate with respect to the substrate. In other words, for thepurposes of this Specification, including the appended claims, the terms“above” and “below” are defined as meaning at greater and lesserdistances, respectively, from an underlying substrate.

FIG. 3 depicts a schematic drawing of a cross-sectional view ofstress-optics phase controller in accordance with the illustrativeembodiment of the present invention. Phase controller 104 includeswaveguide structure 302 and phase-control (PC) element 304. The viewshown in FIG. 3 is taken through line a-a of FIG. 1.

Waveguide structure 302 is that of a surface waveguide having amulti-layer core comprising two silicon nitride layers that are aboveand below a silicon dioxide layer (typically referred to as a“double-stripe” waveguide). The materials and geometry of waveguidestructure 302 are selected to enable it to guide light signals 112,112A, 112B, and 114 such that each light signal has mode field 324. Inthe depicted example, mode field 324 is a slightly elliptical mode fieldcentered at core center C1 having a width, D1 x, of 1.62 microns in thex-direction and a width, D1 y, of 1.56 microns in the y-direction. Modefield 324, therefore, has an average mode-field diameter, D1, of 1.58microns.

It should be noted that, although the illustrative embodiment includes awaveguide structure having a multi-layer core that defines adouble-stripe waveguide, the present invention is suitable for use withvirtually any waveguide structure that includes suitable core structureand/or materials. Other waveguide structures suitable for use inembodiments of the present invention include, without limitation:single-layer-core waveguides whose cores include a dielectric material(e.g., silicon nitride, doped or undoped silicon oxide, siliconoxynitride, etc.), a semiconductor or semiconductor compound, (e.g.,silicon, a compound semiconductor, silicon carbide, silicon germanium,etc.), and the like; and multi-layer-core waveguides whose corescomprise one or more dielectric materials, one or more semiconductormaterials, combinations of dielectric and semiconductor materials, andthe like.

PC element 304 is analogous to PC element 204 described above and withrespect to FIG. 2; however, PC element 304 has several differences ascompared to PC element 204 that enable it to more efficiently impart aphase change on light signal 112B. Chief among these differences arethat the shape of PC element 304 substantially matches the shape of modefield 324 and its stress-concentration points reside at or belowfeatures of core 308.

FIG. 4 depicts operations of a method suitable for forming astress-optic phase controller in accordance with the illustrativeembodiment. Method 400 is described herein with continuing reference toFIGS. 1 and 3, as well as reference to FIGS. 5A-D.

FIGS. 5A-D depict schematic drawings of cross-sectional views of phasecontroller 104 at different points in its fabrication. The views shownin FIGS. 5A-D are taken through line a-a of FIG. 1.

Method 400 begins with operation 401, wherein layer structure 502 isformed on substrate 116. Layer structure 502 includes lower claddinglayer 504, lower core layer 508, central core layer 510, and upper corelayer 512. Lower core layer 508, central core layer 510, and upper corelayer 512 collectively define the layer stack of core 308, which isdisposed on surface 506 of the lower cladding layer.

FIG. 5A depicts nascent waveguide structure 302 after the formation oflayer structure 502.

In the depicted example, lower cladding layer 504 is a layer of silicondioxide having thickness, t2, which is sufficient to mitigate opticalcoupling of mode field 324 into substrate 116. In the depicted example,t2 is approximately 10 microns; however, one skilled in the art willrecognize, after reading this Specification, that other materials and/orthicknesses can be used in lower cladding layer 504 without departingfrom the scope of the present invention.

Each of lower core layer 508 and upper core layer 512 is a layer ofstoichiometric silicon nitride having a thickness of 170 nm. Centralcore layer 510 is a layer of stoichiometric silicon dioxide having athicknesses of 500 nm. Top surface 514 of upper core layer 512 definesplane P1.

At operation 402, core layers 508, 510, and 512 are patterned viaconventional photolithography and reactive-ion etching (RIE) to definethe width, w2, of core 308. In the depicted example, w2 is equal to 1.2microns; however, other core widths can be used without departing fromthe scope of the present invention. Once its width has been defined,core 308 is fully formed and its top surface 514 remains co-incidentwith plane P1.

At operation 403, the exposed regions of lower cladding layer 504 areetched back from surface 506 by over-etch depth d3, thereby definingfield region 516 and spine 530, where field region 516 has thickness,t3, and surface 528. Typically, operations 402 and 403 occur in the sameetch process; however, separate etching steps can be used in theseoperations without departing from the scope of the present invention.

Operations 402 and 403 collectively define ridge 518, which includescore 308 disposed on spine 530 and has a total height of d4. In thedepicted example, the lower cladding is etched back by 4 microns (i.e.,t3 is equal to 6 microns) and the total thickness of core 308 is 0.84microns; therefore, d4 is equal to 4.84 microns.

In the illustrative embodiment depicted in FIGS. 5B through 5D, ridge518 has “vertical” sidewalls (i.e., orthogonal with respect to surface528 in FIG. 5B). In some other embodiments, however, the sidewalls ofthe ridge are not vertical. For example, ridge 518 can have apyramidal-like shape, wherein the sidewalls form an angle with respectto surface 528 that is not 90 degrees. It is within the capabilities ofthose skilled in the art to form ridge 518 with non-vertical sidewalls.

FIG. 5B depicts nascent phase controller 104 after lower cladding 306and core 308 are fully defined.

At operation 404, upper cladding 310 is formed by depositing, inconformal fashion, a layer of silicon dioxide having thickness t1 onsurface 528 and ridge 518. Typically, upper cladding 310 is formed vialow-pressure chemical vapor deposition (LPCVD) using tetraethylorthosilicate (TEOS) as a precursor gas; however, one skilled in the artwill recognize, after reading this Specification, that several conformaldeposition processes suitable for use in the present invention are knownin the prior art. As discussed below, thickness t1 is selected such thatit is less than or equal to d4. In the depicted example, t1 is equal to2 microns; however, other thicknesses for top cladding 310 can be usedwithout departing from the scope of the present invention.

FIG. 5C depicts phase controller 104 after the formation of uppercladding 310.

Because it is a conformal layer, the upper cladding has substantiallythe same thickness (i.e., t1) on each surface on which it is deposited,thereby giving rise to field region 520 and projection 316. Projection316 has outer surface 522 and field region 520 has top surface 524,which defines plane P2. The shape of surfaces 522 and 524 is based onthe topography of lower core 306 and ridge 518.

At operation 405, phase-control element 304 is formed, in conformalfashion, on surface 524. PC element 304 includes electrodes 312-1 and312-2 and piezoelectric layer 314, which is disposed between theelectrodes.

FIG. 5D depicts phase controller 104 after the formation of uppercladding 310.

Each of electrodes 312-1 and 312-2 is an electrically conductivestructure comprising an adhesion layer and a highly conductive layer. Inthe depicted example, each of electrodes 312-1 and 312-2 includestitanium and platinum and has a combined thickness of approximately 100nm.

Piezoelectric layer 314 is a layer of lead zirconate titanate (PZT)having a thickness of approximately 2 microns. In some embodiments,piezoelectric layer 314 comprises a different piezoelectric materialand/or a different thickness.

The formation of PC element 304 on projection 316 and field region 520produces stress-concentration points SP1A, SP1B, SP2A, and SP2B. Stressconcentration points SP1A, SP1B, SP2A, and SP2B function to directstress tensors toward core 308.

In the depicted example, electrode 312-2 extends laterally onto surface526 with width, w3. One skilled in the art will recognize, after readingthis Specification, that the magnitude of w3 can affect the magnitudeand direction of the stress tensors that propagate fromstress-concentration points SP1A, SP1B, SP2A, and SP2B. In someembodiments, w3 is equal to zero (i.e., the electrode stops atstress-concentration points SP2A and SP2B).

Top surface 514 of core 308 defines plane P1, while surface 526 definesthe position of top electrode 312-2 and plane P3, which is below planeP1. Planes P1 and P3 are separated by distance d2. In the depictedexample, d2 is equal to 0.74 microns. In some embodiments, planes P1 andP3 are coplanar.

The shape of PC element 304 is defined by surface 522 of upper cladding310, which wraps around the top and sides of core 308. The shape of modefield 324 is defined by the shape and refractive index configuration ofcore 308. As a result, between stress-concentration points SP1A andSP1B, the shape of PC element 304 is substantially matched to theslightly elliptical shape of optical mode 314 (i.e., separation s1 doesnot vary by more than 20% of D1 along this length).

Furthermore, in some embodiments, the thickness, t1, of upper cladding310 is selected such that the extent of projection 316 is only slightlylarger than the average mode-field diameter, D1, thereby enabling highlyefficient coupling of stress induced by PC element 304 into thematerials that support the mode field. It should be noted, however, thatoptical loss in the waveguide typically increases as the magnitude of t1decreases to approach D1. In some embodiments of the present invention,such as those in which low optical loss is critical, the thickness of t1is increased to mitigate optical loss at the expense of operatingefficiency in phase controller 104.

Still further, as discussed above, embodiments of the present inventionderive significant advantage by ensuring that at least some features ofPC element 304 reside at or below the top of core 308. Furthermore, evenhigher efficiency is achieved for phase controller 104 when plane P3 isequal to or below plane P1.

FIG. 6 depicts a simulation of induced refractive index change versusover-etch depth for a phase controller in accordance with the presentinvention. Plot 600 shows the change in the effective refractive indexof core 308 as a function of over-etch distance, d3 for a waveguidestructure having top cladding thickness, t1, of 3 microns and a PCelement whose piezoelectric layer has a thickness of approximately 2micron.

It can be seen from plot 600 that the maximum change in effectiverefractive index is obtained for an over-etch depth that isapproximately 1 micron greater than the thickness of the top claddingformed upon it. Specifically, for waveguide structures having topcladding thicknesses, t1, of 2.5, 3, and 4, the maximum index changeoccurs for over-etch depths, d3, of approximately 3.5, 4, and 5 microns,respectively.

One skilled in the art will recognize that the relative heights ofplanes P1, P2, and P3 above substrate 116 are based on several factors,including over-etch depth d3, the thickness, t1, of top cladding 310,the thickness of piezoelectric layer 314, and the thicknesses ofelectrodes 312-1 and 312-2.

In some applications, a phase controller that requires very little realestate is desirable. Some phase controllers in accordance with thepresent invention are formed such that they follow a serpentine pathalong the surface of substrate 116 to increase their interaction lengthwithout significantly increasing the chip real estate they require.

FIG. 7 depicts a schematic drawing of a top view of a phase controllerin accordance with a first alternative embodiment of the presentinvention. Phase controller 700 includes waveguide 702 and PC element704.

Waveguide 702 is analogous to waveguide 302 described above; however,waveguide 702 follows a serpentine path through area 706.

PC element 704 is analogous to PC element 304 described above; however,PC element 704 is formed as a rectangular field disposed over waveguide702 in area 706.

It should be noted that, preferably, waveguide 702 is ahigh-index-contrast waveguide that tightly confines its optical modewithin its core, which mitigates optical loss at waveguide turns (i.e.,bending losses). As a result, waveguide 702 can have tighter bendingradii and a higher density path through area 706, which enables a longerinteraction length than a comparable straight waveguide without asignificant increase in propagation loss.

Unfortunately, PC element 704 is characterized by a high capacitance dueto its large area—much of which is ineffective for inducing a phasechange in light signal 112B.

FIG. 8 depicts a schematic drawing of a top view of a phase controllerin accordance with a second alternative embodiment of the presentinvention. Phase controller 800 includes waveguide 702 and PC element804.

PC element 804 is analogous to PC element 704 described above; however,PC element 804 follows the same serpentine path through area 706 aswaveguide 702. As a result, PC element 804 has a much smallercapacitance and, therefore, phase controller 800 can operate at highspeed than phase controller 700.

It is to be understood that the disclosure teaches just one example ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

1. A phase controller comprising: a surface waveguide disposed on asubstrate, the surface waveguide including a lower cladding, a core, andan upper cladding comprising a projection and a field region, whereinthe surface waveguide supports a mode field having a first shape; and aphase-control element disposed on at least a portion of the projection,wherein the PC element comprises a first electrode, a second electrode,and a piezoelectric layer that is between the first and secondelectrodes; wherein the core has a first surface that defines a firstplane, and wherein the field region has a second surface that defines asecond plane, and further wherein the second plane is at or below theheight of the first plane; wherein the PC element includes a firststress-concentration point that is at or below the height of the firstplane; and wherein the PC element has a shape that is substantiallymatched to the shape of the mode field.
 2. The phase controller of claim1 wherein the first electrode is in direct contact with the projectionand the second electrode is not in direct contact with the projection,and wherein the second electrode includes the first stress-concentrationpoint.
 3. The phase controller of claim 1 wherein the first plane andsecond plane are substantially co-planar.
 4. The phase controller ofclaim 1 wherein the first plane and second plane are substantiallyco-planar.
 5. The phase controller of claim 1 wherein the surfacewaveguide has a serpentine shape within a first area on the substrate.6. The phase controller of claim 5 wherein the PC element has aserpentine shape within the first area. 7-18. (canceled)
 19. A phasecontroller comprising: a surface waveguide that is operative forconveying a light signal, the surface waveguide comprising: a firstcladding having a first field region and a spine that projects from thefirst field region; a core that is disposed on the spine, wherein thecore has a first surface that defines a first plane, and wherein thecore and the spine collectively define a ridge; and a second claddingthat includes a second field region and a projection that projects fromthe second field region, wherein the second cladding is conformallydisposed on the first field region and the ridge such that the ridge andthe second cladding collectively define the projection, and wherein thesecond field region has a second surface that defines a second plane,and further wherein the projection has a third surface; and aphase-control (PC) element disposed on the surface waveguide, the PCelement comprising: a first electrode that is in direct contact with thethird surface; a second electrode that is distal to the third surface;and a piezoelectric layer that is between the first and secondelectrodes; wherein the PC element includes a first stress-concentrationpoint.
 20. The phase controller of claim 19 wherein the light signal hasa mode field having a first shape, and wherein the third surface has asecond shape that is matched to the first shape, and wherein the PCelement has the second shape.
 21. The phase controller of claim 19wherein the first stress-concentration point is at or below the firstplane.
 22. The phase controller of claim 19 wherein the PC elementincludes a plurality of stress-concentration points that includes thefirst stress-concentration point, and wherein each of the plurality ofstress-concentration points is at or below the first plane.
 23. Thephase controller of claim 19 wherein the light signal has a mode fieldhaving a first shape, and wherein each of the third surface and the PCelement has a second shape that is matched to the first shape, andfurther wherein the PC element includes a first stress-concentrationpoint that is at or below the first plane.
 24. The phase controller ofclaim 19 wherein the second electrode is disposed on the projection anda first portion of the second field region, the first portion having afirst width.
 25. The phase controller of claim 19 wherein the surfacewaveguide has a serpentine shape within a first area.
 26. The phasecontroller of claim 25 wherein the PC element has a serpentine shapewithin a first area.
 27. The phase controller of claim 19 wherein thecore comprises a first core layer, a second core layer, and a third corelayer that is between the first and second core layers, and wherein thefirst and second core layers comprise silicon nitride and the third corelayer comprises silicon dioxide, and further wherein the third corelayer includes the first surface.
 28. The phase controller of claim 19wherein the surface waveguide is a single-core waveguide, and whereinthe core includes a first core layer that comprises a first materialselected from the group consisting of a dielectric, a semiconductor, anda semiconductor compound.